The Ecology of Dams

Rivers possess a delicate ecology that depends on a regular cycle of disturbance within certain tolerances. The plant and animal communities that inhabit the river and river margins have evolved to adapt to their river's own peculiar pattern of flood and drought, slow and fast current. Dams disrupt this ecology.

There are several types of dams. Check dams are small dams that help control erosion or flooding or, in the dry West, capture runoff to provide water for cattle. Diversion dams divert river water to irrigate crops. Large dams may be built for flood control, irrigation, and/or electrical generation. Flood control dams are often earth dams--made of huge mounds of clay, sand, gravel, and rock--but may instead be made of concrete. Hydroelectric dams are concrete marvels of engineering. This section will examine mostly the large dams: flood control and hydroelectric dams.

Interrupting Natural Cycles

The first effect of a dam is to alter the pattern of disturbances on which the plants and animals of a river depend. Many aquatic animals coordinate their reproductive cycles with annual flood seasons. Every flood is valuable in that it takes nutrients from the land and deposits them in the river, providing food for the stream's residents. Floods also provide shallow backwater areas on vegetated and shaded riversides; the young of many animals depend on these backwaters to protect them from predators too large to swim in these shallow waters.

As an example, a fish on a certain river may only reproduce during April of every year so that its offspring will have abundant food and places to hide. If the normal spring flood never comes because a dam holds the river back, the offspring will be produced during a time when they cannot possibly survive. If the fish breed during the next flood, which may be in July or may be in October, its young will be born during the wrong time of year and will have to contend with the absence of their normal food supply and temperatures for which they are not prepared.

Vegetation, too, depends upon these regular cycles of flood. Quite often, people will decide that they can spare no water at all and no flooding at all will occur in the stream. Or they may have built the dams specifically to stop flooding so they can build houses in the floodplains. When this happens, riparian vegetation, the vegetation bordering the river, changes forever. An example of this may be found in much of the Southwest United States, where enormous floodplains of cottonwood, marsh and grasses have been replaced by dry, barren areas of tamarisk.

Armoring the Riverbed

If the dam is allowed to release water from its reservoir, it will often do so only once in awhile, rather than in the frequent, small floods as are seen in nature. This leads to scouring and armoring of the riverbed. The higher energy of the sudden floods picks up and removes smaller sediments like silt, sand, and gravel, as well as aquatic plants and animals, leafy debris, and large woody debris. Complex sets of habitats are erased. The riverbed below the dam becomes like a pavement of cobbles and loses its value as habitat for plants, macroinvertebrates, and fish.

Another reason that riverbeds become scoured and armored is that dams remove sediment from the river. The river, which now has no sediment, has improved carrying capacity and will pick up sediment from the streambed below the dam. It is much as though the river has been "starved" of its sediment. As in everything else in nature, balance will be achieved one way or the other.

Removing Sediment

What happened to all the sediment in our dammed river? The fast-flowing water became slow-moving water in the reservoir above the dam. This loss of energy caused the river's sediment to drop and and settle behind the dam. This is worrisome: dams are engineered to withstand the force of a certain number of tons of water. They are not engineered to withstand an additional force of tons of wet sediment pressing on their backsides. The muddier the river, the faster this heap of sediment will build up. What happens when it builds up too high? The dam may burst or overflow, killing people and destroying settlements downstream. In effect, a huge man-made waterfall has been constructed, and will remain there until the dam completely collapses.

Can we not remove the sediment from behind dams? Unfortunately, the answer is "No--not yet, we can't." There appears to be no safe and economic way to do it. The Three Gorges Dam on the Yangtze River in China was built with an eye to managing sedimentation. Delicate timing of flow releases was supposed to lead to a stabilization of the sediment backlog within 100 to 150 years. Whether or not that will happen, a recent report admitted that two-thirds of all sediment is being retained behind the dam each year. As a consequence, the river downstream of the dam is being eroded--and the Yangtze River's delta is shrinking. (China's answer to these unexpected levels of sediment retention is to plan yet more dams upstream of the Three Gorges Dam).

What about the river downstream of the dam? Isn't its nice clear, cold water a great improvement--especially in regions like the American Southwest where rivers tend to be muddy? The answer is only "Yes" if you have decided the original ecology of the river doesn't matter. If you only want water to look at and drink, then you will be happy enough. If you want a living river ecosystem filled with fish and birds, you will be sadly disappointed. This cold, clear water will be starved of nutrients and provide little or no habitat for animals. In addition, animals that once used the "muddiness" of the river's water to conceal them from predators are now overly vulnerable to predation, and may quickly go extinct. A river with dams eventually becomes little more than a dead channel of water--and, in the worst case as on the Yangtze River, the estuary that is so critical as a nursery of young fishes as well as a haven for many other species will disappear.

Starving the River

Dams hold back not only sediment, but also debris. The life of organisms (including fish) downstream depends on the constant feeding of the river with debris. This debris includes leaves, twigs, branches, and whole trees, as well as the organic remains of dead animals. Debris not only provides food, it provides hiding places for all sizes of animals and surfaces for phytoplankton and microorganisms to grow. Without flooding and without a healthy riparian zone, this debris will be scarce. Adding to the problem, although debris might come from the river above the dam, it is instead trapped in the reservoir, and never appears downstream. The bottom level of the food web is removed. All in all, the loss of sediment and debris means the loss of both nutrients and habitat for most animals.

Changing Temperatures

Temperature is another problem. Rivers tend to be fairly homogenous in temperature. Reservoirs, on the other hand, are layered. They are warm at the top and cold at the bottom. If water is released downstream, it is usually released from the bottom of the dam, which means the water in the river is now colder than it should be. Many macroinvertebrates depend on a regular cycle of temperatures throughout the year. When we change that, we compromise their survival. For instance, a stonefly may feel the cold temperatures and delay its metamorphosis. This may mean that at a critical lifestage it will be living in the depth of winter rather than in autumn as it should have been.

Stopping Fish Migration

Fish passage is a concern with dams. Many fishes must move upstream and downstream to complete their lifecycles. Dams are often built without fish ladders. When fish ladders are provided, they seldom work as needed. If enough adult fishes do manage to climb above a dam, there remains the issue of their young: how will they get back downstream? Many are killed by predators while they wander in the slow waters of the reservoir above the dam. Many are killed in their fall downward through the dam to the river below. They aren't killed by the fall itself, but by the high levels of nitrogen gas at the base of the dam. In other words, like divers who go too deep, they get the "bends."

There are many fishes that cannot climb dam ladders or leap over low dams. Some of these fishes swim upstream every year to breed, then let the water carry them back downstream. The eggs of pelagic spawners float downstream, too, which is why the adults must swim far upriver to breed. Otherwise, the baby fish would soon end up out to sea.